Technical Field
The present indention relates to a honeycomb filter.
Background Art
Particulates (hereinafter also referred to as PMs or soot) such as soot in exhaust gas discharged from internal combustion engines including diesel engines cause damage to environment and human bodies, and these days people have paid attention to this problem. Since exhaust gas contains tonic gas components such as CO, HC, and NOx, people also worry about the influences of the toxic gas components on the environment and human bodies.
To overcome this problem, various filters having honeycomb structures (honeycomb filters) formed of porous ceramics such as cordierite and silicon carbide have been proposed as exhaust gas purifying apparatus. Such honeycomb filters are connected to internal combustion engines to capture PMs in exhaust gas, or to convert the toxic gas components such as CO, HC, and NOx in the exhaust gas into nontoxic gas.
For enhancing the fuel economy of internal combustion engines and avoiding troubles derived from an increase in the pressure loss during operation, various honeycomb filters have been proposed including those in which the initial pressure loss is lowered by improvement of the cell structure and those in which the rate of increase in the pressure loss is low when a certain amount of PM is accumulated.
Such filters are disclosed for example in WO 2011/042992, WO 2006/035822, JP-T 2009-537741, WO 2004/024294, and U.S. Pat. No. 4,417,908.
WO 2011/042992 discloses a honeycomb filter coated with zeolite as a catalyst with an aim of capturing PMs and purifying Nox. WO 2006/035822 discloses a honeycomb filter in which the porosity is increased and the pore volume ratio of pores with a pore diameter of 10 μm or less is reduced with an aim of lowering the pressure loss after coating with a catalyst.
JP-T 2009-537741 discloses a honeycomb filter in which six exhaust gas introduction cells having a hexagonal cross sectional shape are arranged to surround an exhaust gas emission cell having a hexagonal cross sectional shape, and each exhaust gas introduction cell has a larger cross sectional area than each of the exhaust gas emission cells.
FIG. 21A is a perspective view schematically illustrating a honeycomb filter disclosed in WO 2004/024294. FIG. 21B is a perspective view schematically illustrating a honeycomb fired body forming the honeycomb filter.
As shown in FIGS. 21A and 21B, WO 2004/024204 discloses a honeycomb filter 90 that includes a plurality of honeycomb fired bodies 100 combined with one another with adhesive layers 105 residing therebetween, and an periphery coat layer 106 formed on the periphery of the combined honeycomb fired bodies, wherein the honeycomb fired bodies 100 each include exhaust gas introduction cells 102 each having an open end at an exhaust gas introduction side and a plugged end at an exhaust gas emission side, and exhaust gas emission cells 101 each having an open end at the exhaust gas emission side and a plugged end at the exhaust gas introduction side; the exhaust gas emission cells 101 each have a square cross section perpendicular to the longitudinal direction of the cells; the exhaust gas introduction cells 102 each have an octagonal cross section perpendicular to the longitudinal direction of the cells; and the exhaust gas emission cells 101 and the exhaust gas introduction cells 102 are alternately (in a grid-like pattern) arranged.
Hereinafter, in the explanation of the embodiments of the present invention and background arts, a cell having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side is simply described as an exhaust gas emission cell. Moreover, a cell having an open end at an exhaust gas introduction side and a plugged end at an exhaust gas emission side is simply described as an exhaust gas introduction cell, a first exhaust gas introduction cell, or a second exhaust gas introduction cell.
The term just described as “cell” means both of the exhaust gas emission cell and exhaust gas introduction cell.
Moreover, a cross section perpendicular to the longitudinal direction of cells including exhaust gas introduction cells, exhaust gas emission cells, or the like is simply described as a cross section of the exhaust gas introduction cells, exhaust gas emission cells, or the like.
FIG. 22A is a perspective view schematically illustrating a honeycomb filter disclosed in U.S. Pat. No. 4,417,908. FIG. 22B is a view schematically illustrating an end face of the honeycomb filter.
U.S. Pat. No. 4,417,908 discloses a honeycomb filter 110 in which all cells have the same square cross-sectional shape as shown in FIGS. 22A and 22B. In the honeycomb filter 110, exhaust gas emission cells 111 each having an open end at an exhaust gas emission side and a plugged end at an exhaust gas introduction side are adjacently surrounded fully by exhaust gas introduction cells 112 and 114 each having an open end at the exhaust gas introduction side and a plugged end at the exhaust gas emission side across cell walls 113. In the cross section, a side of the exhaust gas introduction cell 112 faces the exhaust gas emission cell 111 across the cell wall 113, whereas the corners of the exhaust gas introduction cells 114 respectively face the corners of the exhaust gas emission cells 111. Thus, none of the sides forming the cross sections of the exhaust gas introduction cells 114 faces the exhaust gas emission cells 111.